JP2012114163A - Wavelength variable semiconductor laser light source for optical fiber communication - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser light source that can have a narrow line width characteristic and excellent long-term reliability.SOLUTION: A semiconductor laser 10 of the invention is a wavelength variable semiconductor laser light source for optical fiber communication provided with an optical resonator 12. The optical resonator includes a phase rotation unit for turning the phase of laser light oscillated by the optical resonator, and a phase control unit for controlling an oscillation condition about the phase of the laser light. The phase rotation unit is configured by connecting a gain region 16 where laser light is oscillated and a first through port ring resonator 19 having a resonant frequency equal to the oscillation frequency of the optical resonator. In addition, the phase control unit includes a wavelength selection mechanism 17 comprised of an optical filter and a reflector 15 totally reflecting light having passed through the optical filter.

Description

  The present invention relates to a wavelength tunable semiconductor laser light source for optical fiber communication, and more particularly to a wavelength tunable semiconductor laser light source capable of obtaining a narrow linewidth characteristic of about 100 kHz required for a digital coherent method.

  In addition to the fact that networks already occupy a socially important position as infrastructure, the spread of new services such as cloud computing and video distribution has led to an increase in the amount of communication traffic that must be transmitted over the network. ing. For this reason, increasing the speed and capacity of data communication particularly in optical communication networks is a social urgent task.

  The maintenance of the optical fiber communication network corresponding to this occupies most of the installation cost is the installation cost of the optical fiber. For this reason, it is desirable to reduce the installation cost of the optical fiber by using the existing optical fiber communication network as it is. In order to reduce costs, it is desirable to avoid the use of expensive dispersion compensating fibers even when the transmission rate is increased.

  For the reasons as described above, development of optical fiber communication technology using a digital coherent method is in progress. Digital coherent reception is a combination of coherent detection technology and ultra-high-speed digital signal processing technology that encodes the amplitude and phase of an optical carrier on the transmission side and decodes the amplitude and phase after transmission on the reception side. It is a method. That is, by digital signal processing, three values of detection of a phase value encoded by multi-level coding, separation of polarization, and dispersion compensation of an optical fiber transmission line are simultaneously performed. Therefore, it is possible to increase the capacity of the transmission line by using the existing optical fiber communication network as it is without using an expensive dispersion compensating fiber.

  In the digital coherent reception system, a semiconductor laser (LD: Laser Diode) having a narrow spectrum line width (hereinafter simply referred to as a line width) is indispensable in order to stably perform high-speed and large-capacity communication. The reason is the following two points.

  The first point is that the reception sensitivity of the coherent detection of the phase-modulated optical signal is determined by the phase noise of the semiconductor laser, that is, the line width. The second point is to avoid the problem of phase slip. For example, in the case of quadrature phase shift keying (QPSK), when phase noise is mixed during coherent detection, the absolute value of the detected phase value is shifted by π / 2 from the middle of reception. This is a phenomenon in which a fatal reception error occurs.

  More specifically, from the viewpoint of reception sensitivity, a light source having a line width of about 500 kHz or less is used in transmission and reception in the case of the polarization multiplexed quadrature phase shift keying (DP-QPSK). Needed. Further, in the case of the 8-phase phase shift keying (8PSK) or the 16-value quadrature amplitude modulation (16QAM), a light source having a line width of about 100 kHz or less is required for transmission and reception. The From the viewpoint of preventing phase slip, a line width of one-fifth or less is required.

  The line width of the semiconductor laser is inversely proportional to the square of the resonator length when the waveguide loss constituting the resonator is sufficiently small. On the other hand, when the waveguide loss is large, it is inversely proportional to the resonator length. By utilizing this fact, it is conceivable to narrow the oscillation line width by increasing the length of the resonator.

  For example, Non-Patent Document 1 and Non-Patent Document 2 describe semiconductor lasers in which the length of the resonator is increased in order to narrow the oscillation line width. FIG. 11 is an explanatory diagram showing a basic structure of a λ / 4 shift DFB (Distributed Feedback) laser light source 800 described in Non-Patent Document 1.

  The λ / 4 shift DFB laser light source 800 illustrated in FIG. 11 corresponds to a case where the resonator is a semiconductor material. The λ / 4 shift DFB laser light source 800 includes a laser diode 801. In the laser diode 801, a semiconductor active layer functions as a gain region 802, and a wave-like diffraction grating 803 is disposed in the vicinity thereof. A phase shift unit 804 is provided in the diffraction grating 803. This makes it possible to give the laser beam oscillated a strong wavelength selectivity.

  FIG. 12 is an explanatory diagram showing a laser light source 900 described in Non-Patent Document 2. The laser light source 900 in FIG. 12 corresponds to a case where the resonator is a quartz material. A laser light source 900 is provided with a gain region 901 and a reflecting mirror 902 inside a resonator 900 made of a quartz material, and a micro-optics spatial optical system 905 including a lens 903 and a wavelength selector 904. This is the structure provided.

  In addition to Non-Patent Documents 1 and 2, there are the following technical documents related to this. Among them, Non-Patent Document 3 describes a method for calculating the oscillation spectral line width of a semiconductor laser. Patent Document 1 describes a highly reliable and inexpensive wavelength tunable laser light source including a double ring resonator. Patent Document 2 describes a light-emitting element in which stable oscillation characteristics can be obtained by providing a light-emitting element including a multi-stage ring resonator with a piezoelectric element that applies stress to a connection waveguide.

  Patent Document 3 describes a wavelength tunable laser device in which a yield is improved by providing a multi-stage ring resonator with a loop mirror and an asymmetric MZI (Mach-Zehnder interferometer). Patent Document 4 describes an optical semiconductor device in which a wavelength filter and an optical modulator are formed on the same substrate to facilitate wavelength matching.

  Patent Document 5 describes an optical ring filter in which a plurality of ring resonators can be coupled with a directional coupler to couple optical signals having a wide frequency interval. Patent Document 6 describes a laser apparatus that can easily form a Fabry-Perot resonator before cleaving a semiconductor wafer.

Re-specialized WO2005 / 096462 JP 2008-060445 A JP 2009-278015 A JP 2010-027664 A Japanese Patent Publication No. 07-082131 JP 2005-528803 A

IEEE J.Sel. Topics Quantum Electron., Vol.15, pp.514-520, 2009 Electronics Letters, vol.21, No.3, pp113-115, 1985 Ryoichi Ito and Michiharu Nakamura, “Semiconductor Laser [Basics and Applications]”, Baifukan, pages 161-168

  However, using the methods described in Non-Patent Documents 1 and 2, to achieve a line width of 40 kHz, for example, even if the waveguide loss is sufficiently small, the resonator length is 4 mm with a semiconductor material. As mentioned above, it is necessary to make it 9 mm or more with quartz material. In addition, when the waveguide loss is large, it is necessary to increase the length of the resonator several times or more, which causes a problem that the oscillation threshold value is significantly increased.

  In the λ / 4 shift DFB laser 800 shown in FIG. 11, the optical electric field concentrates on the phase shift unit 804 provided near the center of the resonator in order to unify the oscillation longitudinal mode. Then, due to the local concentration of the optical electric field, the refractive index of the phase shift unit 804 changes, and the phase shift effect is reduced.

  As a result, spatial hole burning occurs and the oscillation longitudinal mode becomes unstable. This spatial hole burning tends to occur when the length of the resonator is increased. For this reason, it is difficult to set the resonator length of the λ / 4 shift DFB laser to 1 mm or more. For this reason, the semiconductor laser described in Non-Patent Document 1 cannot achieve a sufficiently narrow line width, and the line width is about 500 kHz.

  In addition, the semiconductor laser 900 shown in FIG. 12 uses a spatial optical system 905 to couple the gain region 901 and the wavelength selection unit 904 to increase the resonator length. This semiconductor laser can temporarily emit a narrow linewidth light emission as compared with the λ / 4 shift DFB laser 800 shown in FIG. 11, but the spatial optical system 905 includes a mechanically movable portion, so that it is vibration resistant. The long-term reliability is low.

  None of the remaining techniques described in Non-Patent Document 3 and Patent Documents 1 to 6 are intended to solve this problem, and it is a matter of course that configurations that can solve this problem are also described. It has not been.

  An object of the present invention is to provide a wavelength tunable semiconductor laser light source for optical fiber communication, which can obtain a narrow linewidth characteristic of about 100 kHz required for a digital coherent system and is excellent in long-term reliability. is there.

  In order to achieve the above object, a wavelength tunable semiconductor laser light source according to the present invention is a wavelength tunable semiconductor laser light source for optical fiber communication including an optical resonator, and the optical resonator oscillates in the optical resonator. A phase rotation unit that rotates the phase of the laser beam; and a phase control unit that controls an oscillation condition for the phase of the laser beam, wherein the phase rotation unit oscillates the laser beam and an oscillation frequency of the optical resonator. The first through-port ring resonator having a resonance frequency equal to is connected and configured.

  As described above, the present invention is configured to have the phase rotation unit configured by connecting the through-port ring resonator to the gain region, and therefore, mechanically movable using the transmission characteristics of the through-port. The semiconductor laser light source device can be shortened without using any part. Accordingly, it is possible to provide a wavelength tunable semiconductor laser light source device having an excellent feature that a narrow linewidth characteristic of about 100 kHz required for a digital coherent method can be obtained and excellent in long-term reliability. Can do.

It is explanatory drawing shown about the structure of the wavelength tunable semiconductor laser light source device which concerns on the 1st Embodiment of this invention. It is explanatory drawing which shows in more detail the structure of the ring resonator shown in FIG. 1, and its periphery. It is explanatory drawing shown about the phase rotation by the ring resonator shown in FIGS. In the semiconductor laser shown in FIG. 1, when there is no effect of phase rotation by the ring resonator, the oscillation condition of the semiconductor laser comprising the Fabry-Perot resonator formed between the first and second reflecting mirrors is illustrated. It is a graph. In the case of considering the effect of phase rotation by the ring resonator in the semiconductor laser shown in FIG. 1, the oscillation condition of the semiconductor laser comprising the Fabry-Perot resonator formed between the first and second reflecting mirrors is as follows. It is an illustrated graph. It is explanatory drawing shown about the analysis model of the generalized semiconductor laser for demonstrating the numerical basis about the component requirement of this embodiment, and its effect. 6 is a graph showing an example of a calculation result of detuning φ dependency of an effective α parameter α_eff in the semiconductor laser shown in FIG. 1. Here, the coupling coefficient k = 0 of the second directional coupler (that is, no monitor output) is set. In the semiconductor laser shown in FIG. 1, in order to obtain a monitor output for tuning the resonance frequency of the ring resonator to the oscillation frequency from the drop port, the coupling coefficient k of the second directional coupler 24 is 0.2. It is a graph shown about effective alpha parameter alpha_eff at the time of doing. It is a figure which shows the structure of the semiconductor laser which concerns on the 2nd Embodiment of this invention. 10 is a graph showing the principle of oscillation wavelength control by the first to third ring resonators shown in FIG. 9. It is explanatory drawing shown about the basic structure of (lambda) / 4 shift DFB laser light source described in the nonpatent literature 1. FIG. It is explanatory drawing shown about the basic structure of the laser light source described in the nonpatent literature 2. FIG.

(First embodiment)
Hereinafter, the structure of the 1st Embodiment of this invention is demonstrated based on attached FIG.
First, the basic content of the present embodiment will be described, and then more specific content will be described.
A wavelength tunable semiconductor laser light source (semiconductor laser 10) according to the present embodiment is a semiconductor laser light source including an optical resonator 12. The optical resonator 12 includes a phase rotation unit 12a that rotates the phase of the laser beam oscillated by the optical resonator, and a phase control unit 12b that controls an oscillation condition for the phase of the laser beam. The phase rotation unit is configured by connecting a gain region 16 that oscillates laser light and a through port ring resonator 19 having a resonance frequency equal to the oscillation frequency of the optical resonator.

  The phase control unit includes a wavelength selection mechanism 17 formed of an optical filter, and a reflecting mirror (second reflecting mirror 15) that totally reflects light that has passed through the optical filter. The resonance frequency of the first ring resonator 18 is configured to be variable. The free spectral region of the first ring resonator 18 is an integral multiple of 50 GHz.

With the above configuration, the semiconductor laser 10 can obtain a narrow linewidth characteristic of about 100 kHz and has excellent long-term reliability.
Hereinafter, this will be described in more detail.

  FIG. 1 is an explanatory diagram showing a configuration of a wavelength tunable semiconductor laser light source device 10 (hereinafter simply referred to as a semiconductor laser 10) according to the first embodiment of the present invention. The semiconductor laser 10 includes an optical resonator including a first optical waveguide 13, a first reflecting mirror 14, a second reflecting mirror 15, a gain region 16, a wavelength selection mechanism 17, and a ring resonator 18 on a substrate 11. 12 is formed. The through port ring resonator 19 is connected to the wavelength selection mechanism 17. The gain region 16 and the ring resonator 18 are the phase rotation unit 12a, and the wavelength selection mechanism 17 and the second reflecting mirror 15 are the phase control unit 12b.

  As for the material system of each of these elements, all the components including the substrate 11 may be compound semiconductors such as an InP (indium phosphide) system and GaAs (gallium arsenide) system, or the substrate 11 and the first light guide. The waveguide 13, the gain region 16, the wavelength selection mechanism 17, and the ring resonator 18 may be a combination of different material systems. In the latter case, a combination of a compound semiconductor and a quartz material, a silicon-based material, or a liquid crystal-based material can be considered.

  The gain region 16 has the same structure as that of an active region of a normal semiconductor laser or semiconductor optical amplifier, and has a buried double hetero structure and a constriction structure of an injection current. Further, a first reflecting mirror 14 is formed at one end of the gain region 16, and the opposite end face is coupled to the optical waveguide 13 with high efficiency without reflection. The first reflecting mirror 14 can obtain an arbitrary reflectance by a dielectric multilayer film or a combination of a dielectric film and a metal film.

  The first optical waveguide 13, the ring resonator 18, and the through port 19 have a structure in which a high refractive index core layer is embedded with a low refractive index cladding layer. The spot size of the optical waveguide 13 is optimized in consideration of the refractive index of each material system so as to couple with the gain region 16 with high efficiency. Here, a spot size converter may be used.

  The wavelength selection mechanism 17 is an optical filter of any structure such as a Fabry-Perot resonator, an etalon resonator, a distributed feedback (DFB), a distributed Bragg reflector (DBR), or a ring resonator. May be used. One end face of the wavelength selection mechanism 17 forms a second reflecting mirror 15, and the remaining one end face is non-reflective and is coupled to the through port 19 with high efficiency.

  In the ring resonator 18, a heater is formed by vapor-depositing a metal such as aluminum, platinum, or chromium on the surface of a ring formed of a SiO2 or SiON material system, and a voltage is applied to the heater from a voltage variable application means 18a. Applied and heated. The refractive index can be changed by changing the applied voltage from the voltage variable applying means 18a. Therefore, the phase of the input light from the first optical waveguide 13 can be changed and output by controlling the voltage input to the heater from the voltage variable application means 18a.

  FIG. 2 is an explanatory diagram showing in more detail the configuration of the ring resonator 18 shown in FIG. 1 and its surroundings. The ring resonator 18 is coupled to the first optical waveguide 13 by the first directional coupler 21. Hereinafter, the gain region 16 side is referred to as the input side. A second optical waveguide 23 is coupled by a second directional coupler 24 on the opposite side of the ring resonator 18 from the first directional coupler 21. Hereinafter, the output side of the second optical waveguide 23 is referred to as a drop port 25.

  The drop port 25 is a monitor output for tuning the resonance frequency of the ring resonator 18. The first optical waveguide 13 and the second optical waveguide 23 are coupled to the ring resonator 18 with the coupling coefficient k by the first directional coupler 21 and the second directional coupler 24.

  FIG. 3 is an explanatory diagram showing phase rotation by the ring resonator 18 shown in FIGS. At the resonance frequency, the phase completely matches between the light traveling straight through the first optical waveguide 13 and the light traveling around the ring resonator 18. However, if the frequency of this light deviates even slightly from the resonant frequency, the light that has made a round around the ring resonator 18 has an effective optical path length longer than the light that has traveled straight through the first optical waveguide 13, and the frequency The amount of phase change increases with respect to the same amount of change.

  For this reason, due to the influence of the light that goes around the ring resonator 18, the phase dependence of the output light from the through port 19 becomes frequency dependent. This is the phase rotation in this embodiment. This effect occurs particularly rapidly in the vicinity of the resonance frequency due to the light interference condition. In order to cause this phase rotation, the light that has traveled straight through the first optical waveguide 13 and the power of the light that has traveled around the ring resonator 18 are designed to be approximately equal. In FIG. 3, the horizontal axis shows the frequency change in the range of resonance frequency ± FSR (Free Spectral Range) / 2, and the vertical axis shows the phase change (phase rotation) relative thereto.

(Principle of narrow line width)
FIG. 4 shows a Fabry-Perot configuration between the first reflecting mirror 14 and the second reflecting mirror 15 when the semiconductor laser 10 shown in FIG. It is the graph which illustrated the oscillation conditions of the semiconductor laser (LD) which consists of a resonator. In FIG. 4, the horizontal axis represents the optical frequency and the vertical axis represents the gain. Note that the effect of the wavelength selection mechanism 17 is ignored here in order to explain the principle of narrowing the line width as the gain of the gain region 16.

  In order for the semiconductor laser 10 to oscillate, it is necessary to satisfy both an oscillation condition for gain and an oscillation condition for phase. The condition that the gain of the gain region 16 is equal to the loss of the entire resonator including the first reflecting mirror 14 and the second reflecting mirror 15 is an oscillation condition for gain. In the case of a Fabry-Perot resonator, the resonator loss is always a constant value, so that the oscillation condition for the gain is a constant value regardless of the optical frequency, as shown by the solid line in FIG.

  The oscillation condition for the phase is that the value obtained by multiplying the reciprocal resonator length by the refractive index of the entire resonator including the gain region 16 and dividing by the oscillation wavelength is an integral multiple of 2π. Since the resonator length is a constant value irrespective of the optical frequency (oscillation wavelength), when the optical frequency is high (oscillation wavelength is short), if the refractive index of the gain region 16 is reduced, the oscillation condition for the phase is set. Will meet. That is, if the carrier density in the gain region 16 increases, the refractive index decreases due to the plasma effect or the like, and the oscillation condition for the phase is satisfied. The dotted line in FIG. 4 indicates the oscillation condition for the phase, and the intersection of the solid line and the dotted line is the oscillation condition of the semiconductor laser 10 that satisfies the conditions for the gain and the phase.

  Next, consider a case where the gain is reduced through gain saturation due to some cause, for example, fluctuation of light intensity (noise). In this case, since the gain is reduced as indicated by the dotted arrow in FIG. 4, the oscillation condition is not satisfied. On the other hand, the oscillation condition for the phase is as shown by the broken line in FIG. Since the intersection of the solid line and the broken line is the oscillation condition for gain and phase, it is necessary to increase the carrier density as described above in order to obtain the gain necessary for oscillation. When the oscillation condition is not satisfied, the carrier density rises because injected carriers are not recombined by stimulated emission. Due to the increase in carrier density, the oscillation condition moves from the intersection of the solid line and the dotted line to the intersection of the solid line and the broken line.

  In FIG. 4, the solid line arrows are frequency fluctuations generated from the intensity fluctuations of the broken line arrows via the α parameter (line width increase coefficient). Here, the α parameter is a parameter determined by the material physical properties of the semiconductor laser, and is defined by the ratio between the refractive index change and the gain change through the change in the carrier density of the active region. In FIG. 4, “frequency chirp (solid arrow) / gain reduction (broken arrow)” is the α parameter.

  FIG. 5 shows a Fabry-Perot structure formed between the first reflecting mirror 14 and the second reflecting mirror 15 in the case where the effect of the phase rotation of the ring resonator 18 is considered in the semiconductor laser shown in FIG. 4 is a graph illustrating the oscillation conditions of the semiconductor laser 10 including a resonator. As in FIG. 4, the horizontal axis represents the optical frequency and the vertical axis represents the gain. That is, FIG. 5 can be said to be obtained by adding the phase change (phase rotation) shown in FIG. 3 to the graph of FIG.

  Due to the effect of the phase rotation of the ring resonator 18, the optical frequency dependence of the resonator phase increases, and the oscillation condition for the phase changes from a dotted line to a broken line. When the gain is reduced through gain saturation due to fluctuations in light intensity, the intersection between the solid line and the broken line in FIG. 5 is the oscillation condition for the gain and phase, so the oscillation condition starts from the intersection between the solid line and the dotted line. Move to the intersection with. That is, since the length of the solid line arrow represents the magnitude of fluctuation of the optical frequency, it can be understood that the line width can be reduced (the α parameter can be reduced) by the effect of the phase rotation of the ring resonator 18.

  FIG. 6 is an explanatory diagram showing a generalized analysis model of the semiconductor laser 50 in order to explain the numerical basis of the configuration requirements and the effects of the present embodiment. Thereafter, in the lines other than the mathematical expressions in this specification, “A with a superscript B (A raised to the B power etc.)” is “A ^ B”, and A is a subscript B with “A”. It is represented by “A_B”. In FIG. 6, the resonator length is L_0, and the reflectances of the first reflecting mirror 14 and the second reflecting mirror 15 are R_1 and R_2, respectively. Here, the first reflecting mirror 14 (reflectance R_1) is a reflecting mirror having no optical frequency dependency, such as a normal semiconductor cleavage, a plane mirror, etc., while the second reflecting mirror 15 (reflectance R_2). Is a reflector having an optical filter characteristic of a ring resonator.

Equation 1 is a Langevin equation for the optical electric field b (t) in the resonator of the ring resonator 18 shown in FIG. Here, ω_0 is the resonant optical frequency, v_g is the group velocity, g is the gain, and F_b (t) is the noise term.

Equation 2 is an oscillation condition equation indicating an oscillation condition for a gain in the resonator in the ring resonator 18 shown in FIG. The right side shows the resonator loss, and the left side shows the gain. The oscillation condition is that the average values of the right side and the left side are equal.

Equation 3 is an oscillation condition equation indicating the oscillation conditions for the phase in the ring resonator 18 shown in FIG. The oscillation condition is that the sum of the resonator phase considering the normal α parameter and the phase rotation by the ring resonator of R_2 is an integral multiple of 2π.

The oscillation spectral line width of the semiconductor laser 10 can be calculated according to the contents described in Non-Patent Document 3 described above. First, Equations 2 and 3 are expanded by Δω to obtain Equations 4 and 5. Here, Δω = ω−ω_0.

  Next, Equation 1 is expanded by small signal approximation with respect to the fluctuation of the complex electric field and the number of carriers, and each is subjected to Fourier transform, and the spectral density function of the phase noise is obtained from the correlation function of the noise term F_b (t) of the Langevin equation. From the spectral density function and Equations 4 and 5, Equations 6 to 8 are finally obtained.

Equation 6 is an expression showing the definition of ν. Equation 7 is an expression showing the definition of ν_0. Equation 8 is an equation in which the oscillation conditions of the ring resonator 18 shown in FIGS. 2 to 3 are rewritten using these ν and ν_0. Here, for simplicity, detuning (the difference between the laser oscillation frequency and the ring resonance frequency) is defined by the phase difference φ. K is the resonator loss, Li is the ring length of the i-th ring resonator among the plurality of ring resonators, and v_g0 and v_g1 are the group velocities of the active region and the optical waveguide, respectively.

  In Equations 6 to 8, K is the resonator loss, n_sp is the inversion distribution parameter, b_0 ^ 2 is the number of photons, L_i is the ring length of the i-th ring resonator when considering a plurality of ring resonators, v_g0 and v_g1 Are the group velocities of the active region and the optical waveguide, respectively.

  Equation 7 is an equation representing the line width of a conventional semiconductor laser. When the α parameter is large, the line width is proportional to the square of the α parameter. Expression 6 is an expression representing the line width of the semiconductor laser 10 according to the present invention, and the line width is inversely proportional to the square of the coefficient F of Expression 8.

  In Equation 8 representing the coefficient F, the second term is always a positive value as long as the refractive index is positive, but the third term can be either positive or negative depending on the value of the detuning φ. In some cases, F> 1 and the line width becomes narrower. It can also be seen that the line width becomes narrower as the ring length L_i of the ring resonator 18 is longer and the number of ring resonator optical filters inside the resonator is larger.

Equation 9 is a definition of the transfer function R (φ) of the through port 19. Equation 10 is an equation representing the coefficient F shown in Equation 8 using the transfer function R (φ) shown in Equation 9.

  FIG. 7 is a graph showing an example of a calculation result of the detuning φ dependency of the effective α parameter α_eff in the semiconductor laser 10 shown in FIG. Here, the effective α parameter α_eff is a value corresponding to “frequency chirp (solid arrow) / gain reduction (broken arrow)” in FIG. 5, and is calculated using Equation 7 for Δν_0 of Equation 6 including the coefficient F. Value. Here, α = 3, the coupling coefficient k of the first directional coupler 21 is 0.5, and the coupling coefficient k of the second directional coupler 24 is 0 (that is, there is no monitor output).

  From the graph shown in FIG. 7, it can be seen that the effective α parameter α_eff becomes the minimum value when the detuning φ = 0. When φ = 0, as can be seen from the second term of Equation 10, the value of the effective α parameter α_eff is about 1/3 of α due to the effect of phase rotation by the ring resonator.

FIG. 8 shows the coupling of the second directional coupler 24 in order to obtain a monitor output from the drop port 25 for tuning the resonance frequency of the ring resonator 18 to the oscillation frequency in the semiconductor laser 10 shown in FIG. It is a graph shown about effective alpha parameter alpha_eff when coefficient k = 0.2. Even when the coupling coefficient k is set to a relatively large value of k = 0.2, the influence on the line width is small, and the value of the effective α parameter α_eff is α when detuning φ = 0 as in FIG. It becomes about 1/3 of. Here, the ITU-T (International Telecommunication Union Telecommunication) which can easily apply the FSR (Free Spectral Range) of the ring resonator 18 to actual optical communication.
Standardization Sector) It can be easily set to be an integral multiple of 50 GHz which is the grid interval.

  In the present invention, instead of the conventional method by increasing the length of the resonator, a phase rotation unit comprising a through port of a ring resonator is inserted inside the resonator. This phase rotation reduces frequency fluctuations due to changes in carrier density, so that the α parameter can be effectively reduced and a narrow line width can be realized. The principle of operation of the present invention is also applied to a semiconductor laser having any wavelength control mechanism such as a Fabry-Perot resonator, an etalon resonator, a distributed feedback structure (DFB), a distributed Bragg reflector (DBR), a ring resonator, etc. Thus, narrow line width characteristics can be realized.

  In this embodiment, since the transmission characteristic of the through port of the ring resonator is used, the semiconductor laser light source device can be shortened in length, that is, downsized. As a result, it is possible to eliminate the need for reducing the loss of the waveguide. Furthermore, since this semiconductor laser light source device has the same structure as a ring resonator for wavelength control, it can be applied to a wavelength tunable light source. At this time, the influence of the coupling coefficient of the second directional coupler for obtaining a monitor output for tuning the resonance frequency of the ring resonator to the oscillation frequency is not a problem even if not particularly considered.

(Second Embodiment)
In the semiconductor laser 100 according to the second embodiment of the present invention, the wavelength selection mechanism is configured by one or a plurality of second ring resonators, and the reflecting mirror is configured by a loop mirror. In addition, the adjustment control means 110 is configured so that the resonance frequencies of the plurality of second ring resonators are different from each other, and these resonance frequencies are controlled in cooperation according to an oscillation frequency commanded by an external input. Provided.

Also with this configuration, not only the same effect as in the first embodiment can be obtained, but also a narrow line width characteristic of 20 kHz or less can be obtained in a wider frequency band.
Hereinafter, this will be described in more detail.

  FIG. 9 is a diagram showing a configuration of the semiconductor laser 100 according to the second embodiment of the present invention. In the semiconductor laser 100, the wavelength selection mechanism of the phase controller in the semiconductor laser 10 according to the first embodiment described above is configured by the first to third ring resonators 101 to 103, and the second reflecting mirror (total reflection) The mirror) is constituted by a loop mirror 105.

  That is, the first to third ring resonators 101 to 103 and the loop mirror 105 operate as the phase control unit 112b, and the fourth ring resonator 104 and the gain region 116 operate as the phase rotation unit 112a. Voltage variable applying means 101a to 104a for adjusting the applied voltage are attached to each of the first to fourth ring resonators 101 to 104, and the voltage variable applying means 101a to 103a are external (user or other). Also included is an adjustment control means 110 that adjusts the applied voltage in coordination with the oscillation frequency commanded by the input from the device.

  The semiconductor laser 100 includes an optical waveguide 113 made of a SiO 2 and SiON material system, which is formed on a silicon substrate 111 by a combination of plasma CVD or sputtering, etc., etching processing, annealing heat treatment process, and the like. An optical resonator 112 including first to fourth ring resonators 101 to 104 having a waveguide structure and a loop mirror 105 is formed. The optical resonator 112 further includes a first reflecting mirror 114, a phase control region 115, a gain region 116, and a directional coupler 121 mounted on the silicon substrate 111.

  The optical waveguide 113 or the like has a structure in which a core layer made of a high refractive index SiON material is embedded with a cladding layer made of a low refractive index SiO 2 material. The difference in refractive index between the core layer and the cladding layer is about 6%. The spot size of the optical waveguide 113 is optimized in consideration of the refractive index, layer structure, etc. of each material system so as to be coupled with the gain region 16 with high efficiency. Here, a spot size converter may be used.

  The gain region 116 has the same structure as that of an active region of a normal semiconductor laser or semiconductor optical amplifier, and a semiconductor light having a wavelength of 1.55 μm made of an InP-based material having a buried double hetero structure and a constriction structure of an injection current. It is an amplifier. The gain region 16 has a length of 500 μm, and a quantum well structure or a strained quantum well structure may be used for the active layer. The value of the α parameter (line width enhancement factor) determined by the material structure of the active layer is approximately 3.

  A first reflecting mirror 14 having a reflectance of about several percent is formed on the end face of the gain region 116 on the light output side. The first reflecting mirror 114 can obtain an arbitrary reflectance by a dielectric multilayer film or a combination of a dielectric film and a metal film. A dielectric film is formed on the opposite side of the gain region 116 to be a non-reflection end face, and is coupled to the optical waveguide 113 with high efficiency.

  The gain region 116 is hybrid-integrated on the silicon substrate 111 by a passive alignment mounting technique. If the passive alignment mounting technique is used, the optical axis alignment between the gain region 116 and the optical waveguide 113 is possible with an error of sub-μm or less, and high-efficiency coupling can be achieved.

  Here, all the components such as the first to fourth ring resonators 101 to 104 and the loop mirror 105 are integrated on the silicon substrate 111. On the other hand, since the gain region 116 and the silicon substrate 111 are firmly fixed by the bump structure made of Au by the passive alignment mounting technique, the semiconductor laser according to the second embodiment has long-term stability and reliability. Is secured.

  The first to third ring resonators 101 to 103 inserted in the resonator formed between the first reflecting mirror 114 and the loop mirror 105 select the wavelength by adjusting the applied voltage by the adjustment control means 110. Acts as a mechanism. That is, any wavelength that satisfies the phase condition of the resonator formed between the first reflecting mirror 114 and the loop mirror 105 is operated as a wavelength selection mechanism via the adjustment control means 110. If the ring resonators 101 to 103 are selected, the oscillation wavelength (oscillation frequency) can be controlled.

  FIG. 10 is a graph showing the principle of oscillation wavelength control by the first to third ring resonators 101 to 103 shown in FIG. FIG. 10 shows light having a transmittance (defined by the reciprocal of the loss of the entire resonator) of the entire resonator including the first reflecting mirror 114, the loop mirror 105, and the first to third ring resonators 101 to 103. The frequency dependence is shown in a graph. In FIG. 10, Fabry-Perot resonance characteristics are resonance modes formed by the first reflecting mirror 114 and the loop mirror 105.

  As shown in FIG. 10, the FSR (Free Spectral Range) of the first to third ring resonators 101 to 103 is designed to adjust the optical frequency so that the ITU-T (International Telecommunication Union Telecommunication Standardization Sector) can be used. ) It is configured to be slightly different from each other around 100 GHz which is twice the grid interval. The adjustment loss is minimized by adjusting the optical frequency at which the transmittance of the three ring resonators 101 to 103 is the same by the adjustment control means 110, and the optical frequency is calculated from the Fabry-Perot resonance mode. You can select and oscillate and control the wavelength.

  Here, the FSRs of the first to third ring resonators 101 to 103 are configured to have different values from each other because of the so-called vernier effect that is the same principle as the caliper vernier scale. This is to widen the wavelength variable range in which the wavelength can be controlled even when the change amount is small. For example, when there are two ring resonators and the FSR differs by 10%, the wavelength variable width that can be controlled with the same amount of change in refractive index is 10 times.

  In order to set the FSR to about 100 GHz, the curvature radii of the first to third ring resonators 101 to 103 are about 300 μm. In addition, the coupling coefficient k of the directional couplers of these ring resonators is set to 0.35. On the other hand, the fourth ring resonator 104 constituting the phase rotation unit has FSR = 100 GHz and a directional coupler coupling coefficient k = 0.5.

  The phase control region 115 adjusts the phase of the Fabry-Perot resonance mode of the resonator formed between the first reflecting mirror 114 and the loop mirror 105, and the resonance frequency of the Fabry-Perot resonator and three ring resonances. This is for matching the resonance frequency of the vessel. In this phase adjustment, a heater made of Pt or the like is provided with a clad layer having an appropriate thickness sandwiched between the core layers of the phase control region 115 so as not to cause light absorption loss, and the refractive index changes by heating the heater. This is done by controlling the refractive index of the SiON core layer using the effect.

  In addition, the first to fourth ring resonators 101 to 104 also have a heater formed immediately above the core layer, and change the refractive index of the core layer made of the SiON material by utilizing the thermal effect, thereby changing the oscillation frequency and the ring. Control resonance frequency. That is, the resonance frequency of the fourth ring resonator 104 constituting the phase rotation unit is tuned with respect to the oscillation frequency controlled according to the above-described principle. This tuning may be performed by fixing the resonance frequency of the fourth ring resonator 104 and changing the phase condition of the Fabry-Perot resonator by the phase control region 115.

  As can be seen from FIG. 7, the line width is minimized when the resonance frequency of the fourth ring resonator 104 is tuned to be equal to the oscillation frequency. When the line width is calculated by the same formula as in the first embodiment using the above-described structural parameters with respect to FIG. 9, a narrow line width characteristic of 20 kHz or less is obtained. Further, the above tuning method based on the principle of wavelength control shown in FIG. 10 is valid for the oscillation wavelengths of all the channels (for example, ITU-T grid) of the wavelength tunable laser.

  Therefore, the semiconductor laser 100 having the structure shown in FIG. 9 can realize a small and highly reliable wavelength tunable laser having a narrow linewidth characteristic of 20 kHz or less in all channels.

  The present invention has been described with reference to the specific embodiments shown in the drawings. However, the present invention is not limited to the embodiments shown in the drawings, and any known hitherto provided that the effects of the present invention are achieved. Even if it is a structure, it is employable.

  About each embodiment mentioned above, it is as follows when the summary of the novel technical content is put together. In addition, although part or all of the said embodiment is summarized as follows as a novel technique, this invention is not necessarily limited to this.

(Supplementary note 1) A wavelength tunable semiconductor laser light source including an optical resonator,
The optical resonator is
A phase rotation unit that rotates the phase of the laser beam oscillated by the optical resonator;
A phase control unit that controls oscillation conditions for the phase of the laser beam,
The phase rotation unit is
A gain region for oscillating the laser beam;
A laser light source comprising: a first through-port ring resonator having a resonance frequency equal to the oscillation frequency of the optical resonator.

(Supplementary Note 2) The phase control unit is
A wavelength selection mechanism comprising an optical filter;
The laser light source according to appendix 1, further comprising a reflecting mirror that totally reflects light that has passed through the optical filter.

(Supplementary note 3) The laser light source according to supplementary note 1 or supplementary note 2, wherein a resonance frequency of the first ring resonator is variable.

(Supplementary note 4) The laser light source according to any one of supplementary notes 1 to 3, wherein a free spectral region (FSR) of the first ring resonator is an integer multiple of 50 GHz.

(Supplementary Note 5) The wavelength selection mechanism is constituted by one or a plurality of second ring resonators,
The laser light source according to appendix 2, wherein the reflecting mirror is configured by a loop mirror.

(Additional remark 6) The resonance frequency of the plurality of second ring resonators is configured to be different from each other, and the resonance frequency is adjusted in cooperation with the oscillation frequency commanded by an external input. The laser light source according to appendix 5, wherein a control means is provided.

  The present invention can be widely used as a light source for optical fiber communication. In particular, it is suitable for optical fiber communication by a digital coherent method.

10, 50, 100 Semiconductor laser 11 Substrate 12, 112 Optical resonator 12a, 112a Phase rotation unit 12b, 112b Phase control unit 13 First optical waveguide 14, 114 First reflecting mirror 15 Second reflecting mirror 16, 116 Gain region 17 Wavelength selection mechanism 18 Ring resonator 18a, 101a to 104a Variable voltage applying means 19 Through port 21 First directional coupler 23 Second optical waveguide 24 Second directional coupler 25 Drop port 101 First Ring resonator 102 second ring resonator 103 third ring resonator 104 fourth ring resonator 105 loop mirror 110 adjustment control means 111 silicon substrate 113 optical waveguide 115 phase control region 121 directional coupler

Claims (6)

A tunable semiconductor laser light source including an optical resonator,
The optical resonator is
A phase rotation unit that rotates the phase of the laser beam oscillated by the optical resonator;
A phase control unit that controls oscillation conditions for the phase of the laser beam,
The phase rotation unit is
A gain region for oscillating the laser beam;
A laser light source comprising: a first through-port ring resonator having a resonance frequency equal to the oscillation frequency of the optical resonator. The phase control unit is
A wavelength selection mechanism comprising an optical filter;
The laser light source according to claim 1, further comprising a reflecting mirror that totally reflects light that has passed through the optical filter.   The laser light source according to claim 1 or 2, wherein a resonance frequency of the first ring resonator is variable.   4. The laser light source according to claim 1, wherein a free spectral range (FSR) of the first ring resonator is an integral multiple of 50 GHz. 5. . The wavelength selection mechanism is constituted by one or a plurality of second ring resonators,
The laser light source according to claim 2, wherein the reflecting mirror is a loop mirror.   The resonance frequency of the plurality of second ring resonators is configured to be different from each other, and adjustment control means is provided for controlling the resonance frequencies in cooperation with each other according to the oscillation frequency commanded by an external input. The laser light source according to claim 5, wherein

Images